1. Field of Invention
The present invention relates to a multi-layer circuit board. More particularly, the present invention relates to a coreless thin substrate with embedded circuits in dielectric layers and a method for manufacturing the same.
2. Description of Related Art
Multi-layer circuit boards are used in various electronic products for the connection of electrical signal transmission, power supply, and grounding. With the miniaturization of the electronic products, it is required that the multi-layer circuit boards become thinner, and circuits on the multi-layer circuit boards become denser. Generally, the multi-layer circuit boards are classified into lamination circuit board and build-up circuit board according to the manufacturing processes.
In the lamination process, a plurality of circuit boards formed with a core layer and surface circuit layers are combined to a multi-layer circuit board through a thermal lamination step, and an insulating layer that is not completely polymerized is disposed between the circuit boards. In the build-up process, a circuit board including a core layer and surface circuit layers is printed layer by layer to form insulating layers and electroplated to form circuit layers. Both the lamination process and the build-up process need a circuit board including a core layer as a base unit for lamination or a carrier for build-up, so the multi-layer circuit board has a great thickness. Moreover, as the surface of the core layer is not planar, the circuit layer formed on the core layer is also not planar. And after multiple layers of the core layer, the insulating layers, and the circuit layers are formed layer by layer, the thickness of the multi-layer circuit board cannot be controlled, and the surface of the multi-layer circuit board is not planar.
Taiwan Patent No. I236324 entitled “Insulating Layer Structure of Circuit Board and Method for Forming Circuit Board Using the Insulating Layer” has disclosed relevant laminated multi-layer circuit boards and build-up circuit boards. Moreover, Taiwan Patent Publication No. 585016 entitled “Build-Up Process of Multi-Layer Printed Circuit Board and Structure thereof” has disclosed a multi-layer printed circuit board fabricated with a mixture of the build-up process and the lamination process. However, in the aforementioned known multi-layer circuit board structures, a core layer is required, and the circuits are protruded from the surface of the core layer, which are disadvantageous to the flattening and planarization of the circuit board.
FIG. 1 is a schematic sectional view of a conventional laminated multi-layer circuit board. The multi-layer circuit board 100 is formed by a plurality of core substrates 110 through thermal lamination, and an insulating layer 120 is disposed between the core substrates 110. Each of the core substrates 110 includes a core layer 111 and a plurality of circuit layers 112, 113 protruding from an upper surface and a lower surface of the core layer 111 respectively. In a core substrate 110, the circuit layers 112, 113 of the same core substrate 110 are electrically connected via a conducting hole 114. During the thermal lamination, the insulating layer 120 is a layer of incompletely polymerized material (i.e., Stage B), so as to bond the core substrates 110, and the insulating layer 120 flows to fill the gaps among the inner circuit layers 112, 113 and the conducting holes 114. However, the thickness of the insulating layer 120 for electrically insulating different core substrates 110 cannot be controlled. In addition, the conventional thermal lamination process merely achieves the mechanical bonding of different core substrates 110. After the thermal lamination, different core substrates 110 are not electrically connected, and subsequent through-hole formation process is required to form an appropriate through hole 130, which penetrates the core substrates 110 and the insulating layer 120. Moreover, a metal layer is electroplated in the through hole 130, so as to electrically connect the core substrates 110. Finally, a solder mask 140 is formed on each of the outside surfaces of the core substrates 110, so as to form the multi-layer circuit board 100. As the manufacturing flow of the conventional laminated multi-layer circuit board 100 is complicated, the manufacturing cost is high, and the thickness of the circuit board 100 is great.